1. Field of the Invention
The present invention relates generally to electromagnetic field measuring devices and more particularly to an instrument which utilizes a Luneberg lens mounted in the radome of a missile for the measurement of an incident electromagnetic field.
2. Description of the Prior Art
There is currently a need in target seeking missiles for an electromagnetic field sensing apparatus which will accurately measure incoming electromagnetic wave fronts. Such an apparatus should have the capability of detecting incoming electromagnetic wave fronts such as radar/microwave signals as well as determining the angle of arrival of these incoming electromagnetic wave fronts without the scattering of the wave front which is normally associated with conventional electromagnetic field sensing apparatus such as metallic antennas. In addition, such an apparatus should have the capability of detecting the polarization of an incident electromagnetic field.
Target seeking missiles incorporating electromagnetic field sensors are also required to operate at high speeds. Aerodynamic considerations, in turn, generally require the use of an electrically transparent radome for enclosing the electromagnetic field sensor in a target seeking missile to provide protection of the sensor from meteorological events such as wind and water. In the case of military equipment, such as target seeking missiles, protection is also required from the concussive effects of nearby guns or the blast from near hits of an exploding projectile.
Target signals passing through a radome will be incident on the inside surface of the radome inner wall or bulkhead of the missile resulting in a substantial portion of the incident energy being reflected by the bulkhead. The reflected signal will probably intercept the antenna which results in the antenna receiving the incoming electromagnetic field energy as well as the reflection of this electromagnetic field. A wide beam-width antenna, which is generally preferred in tracking systems for target seeking missiles, will detect both incoming and reflected electromagnetic energy and the resulting interference between them will lead to a severe degradation of the radiation patterns. This, in turn, results in the tracking system of the missile being unable to effectively locate and destroy enemy targets.
U. S. Pat. No. 5,173,699 which issued Dec. 22,1992 to Barr et. al. partially solves this problem of direct reflection from the internal wall of the missile radome by utilizing a radome inner wall fitted with surface wave absorbent material. In addition, the remaining space between the antenna, which is metallic, and the radome is at least partially filled with radar absorbent material so as to at least partially suppress radar signals reaching the antenna by other than a direct path through the radome.
While partially reducing the effects of reflected radiation, metallic antenna, such as the antenna of U.S. Pat. No. 5,173,699 develop surface currents which are induced by incident electromagnetic fields being sensed, thereby creating scattered electromagnetic fields. These scattered electromagnetic fields disturb the incident electromagnetic fields being sensed reducing the accuracy of the electromagnetic field measurements. Further these scattered electromagnetic fields provide a signature for enemy detectors in a hostile environment. Further, the transmission lines for these metallic antenna enhance the above problems in that the transmission lines are also metallic.
The antenna assembly used by target tracking missiles generally comprises pairs of antenna elements for tracking a target in azimuth and elevation and generally referred to as monoplause tracking radar. As a result of degradation of the types discussed above the tracking response of a pair of antenna for a monoplause tracking radar will suffer perturbation to the desired monotonic (within the operating field of view), and approximately linear, difference characteristic. Minor perturbations are manifested as ripples on the tracking response which cause a localized tracking error; major perturbations result in a reversal of the tracking curve (difference characteristic) and lead to ambiguous target bearing information.
A need therefore exists for a tracking radar for use in a target seeking missile which provides a long-sought solution to the problem of accurately tracking a target by, for example, determining the angle of arrival of an incoming electromagnetic wave reflected from the target.